1. Field of the Invention
The present invention relates to a proximity sensor to be used for applications to detect an approaching metallic material and output an ON signal, to detect the distance from such a metallic material, and so forth. More specifically, the invention relates to an amplifier separate type proximity sensor wherein a circuit for outputting a signal representing a detection result is contained in a case different from one for the detecting coil.
2. Description of the Background Art
Generally, a proximity sensor comprises a high frequency oscillating circuit equipped with an LC resonance circuit, a detecting circuit for detecting oscillation amplitude of this oscillating circuit, a controlling circuit for generating a signal representing a detection result by use of the output from the detecting circuit, an output circuit for outputting a signal representing the detection result, and the like. Especially, a sensor of a type that outputs a signal representing a distance to a metallic material as the detection result, wherein functions of the controlling circuit and the output circuit increase in number and the circuit scale thereof becomes large, is structured in many cases as an amplifier separate type sensor in which at least circuits after the detecting circuit are arranged in a case different from one for a detecting coil.
FIG. 4 shows an example of an amplifier separate type proximity sensor according to the prior art. The proximity sensor shown therein includes a detecting coil 201, an initial stage circuit portion 202 including an oscillating circuit and the similar in its head portion 101A, while circuits of the detecting circuit, controlling circuit, output circuit and the like (collectively shown as a main circuit portion 203 in the figure) in its amplifier portion 102A, and the head portion 101A and the amplifier portion 102A are connected with a cable.
In the structure mentioned above, a capacitor that as well as the detecting coil 201 configures a resonance circuit is included in the oscillating circuit of the initial stage circuit portion 202. Invariables of the capacitor and resistance and the similar are adjusted according to the invariable of the detecting coil so that the oscillating circuit carries out oscillation at a specific frequency.
And the initial stage circuit portion 202 further includes a memory to which parameters and the like are memorized in order to compensate the distance to a detection object and oscillation amplitude so that their relation should be a proportional relation. The main circuit portion 203 compensates the oscillation amplitude of the oscillating circuit on the basis of information in this memory and outputs the oscillation amplitude, and also outputs a digital signal representing presence or absence of an object through comparison of this output value with a specific threshold value.
FIG. 5 shows another structure of an amplifier separate type proximity sensor. In this proximity sensor, its head portion 101B wherein only a detecting coil 201 is arranged, and its amplifier portion 102B wherein an initial stage circuit portion 202 and a main circuit portion 203 are arranged are connected with a cable. The initial stage circuit portion 202 in this case is also equipped with an oscillating circuit and a memory to which invariables are set according to each kind of the detecting coil 201, accordingly, the main circuit portion 203 may output a voltage signal proportional to the distance to a detection object, on the basis of information in the memory.
In the proximity sensor of the structure shown in FIG. 4, wherein the detecting coil 201 and the initial stage circuit portion 202 set according to the characteristics thereof are contained in the head portion 101A, it is possible to make identical the type of the amplifier portion 102A irrespective of the kind of the detecting coil 201. As a consequence, control at manufacturers' places may become easy, and users may reduce costs.
However, in the case when the initial stage circuit portion 202 is included also in the head portion 101A, the size of the head portion 101A become large, in some cases, it is impossible to install the head portion 101A in a place that users desire (for example, in a robot hand portion). And further, such an electronic part as a memory is arranged at the head portion 101A, accordingly, it is difficult to install the head portion 101A under high temperature environments, and this lead to low environmental resistance, which has been a problem in the prior art.
On the other hand, according to the structure shown in FIG. 5, only the detecting coil 201 has to be contained in the head portion 101B, consequently, it is possible to make the head portion 101B compact and to install it in easy manners. However, the detecting coil 201 at the head portion 101B and the initial stage circuit portion 202 at the amplifier portion 102B have to be corresponded according to kinds and characteristics of the detecting coil 201, therefore, it is necessary to prepare the amplifier portion 102B for each type of the head portion 101B. For this reason, if the head portion 101B gets faulty and must be exchanged with one of a different type, then the amplifier portion 102B must also be exchanged, which leads to increased costs at users. And at manufacturers' side, the head portion 101B and the amplifier portion 102B must also be controlled on one to one basis, which increases burdens and deteriorates manufacturing efficiency too, which has been another problem in the prior art.
While, as shown in FIG. 5, in the case where a cable is arranged between the detecting coil 201 and the initial stage circuit portion 202, the electrostatic capacity that this cable has will function as part of the capacitor of the resonance circuit. Therefore, if the length of the cable is changed, the oscillation frequency fluctuates accordingly, so it is preferable to fix the cable length between the head portion 101B and the amplifier portion 102B. And in order to minimize the temperature curve and the noise influence between the head portion 101B and the amplifier portion 102B, it is preferable to make the cable as short as possible. Nevertheless, environment where this kind of a sensor is installed varies greatly, therefore, if the cable length is fixed short, it becomes difficult to install the amplifier portion 102B in some environmental conditions, which has been a still further problem in the prior art.